The invention relates generally to the field of drafting equipment and particularly to architectural rendering equipment and the like.
All architects who do any drawing have a drafting table with a standard device called a parallel bar or parallel rule. Because of a crisscrossed cable and pulley arrangement, the parallel rule remains parallel when moved up or down to allow the drawing of horizontal parallel lines, and with the addition of a triangle rule layed firmly against the edge of the parallel rule, the drawing of vertical lines. Thus the parallel rule is one of the most elementary drafting tools. Conventional parallel rule assemblies have been marketed for many years, for example, by Mayline Corporation of Sheboygan, Wis.
In most perspective drawings, vertical edges remain as true verticals; therefore, these edges can be constructed using a conventional parallel rule. The construction of nominally horizontal lines is an entirely different matter, because they must radiate from a distant point which may be 10 feet or more away and thus they will never be parallel. As the location of these points are specified by a geometrical method, the resulting radius lines must diverge in a predictable fashion, creating a controllable set of angles; from these angles comes the illusion of horizontal edges of planes tapering into the distance.
Large architectural renderings frequently involve the use of cumbersome, inconvenient or expensive equipment to draw perspective lines having vanishing points on the order of 15 feet. Incredible as it may seem to laymen, the conventional practice has been to use a 15 foot long pivoted ruler for doing 15 foot perspective drawings. The alternatives are the purchase of expensive automatic perspective drawing machinery such as that currently offered for example by Dietzgen Corporation of Chicago, Ill., or the use of a transparent sheet having preruled perspective lines of the required vanishing point placed on a drawing board illuminated from behind, necessitating the use of tracing paper for the drawing and stocking various sizes of preruled sheets. Consequently, many architects have chosen to routinely send out large perspective drawings to professional rendering companies.